This invention relates to the field of gaseous component presence detection and measurement by way of electrical conductivity changes.
The detection of low-level gaseous contaminants in the atmosphere and in other gaseous environments continues to be an important and evolving chapter in the measurement art. In addition to the expected chemical warfare and defensive or protective apparatus implications of this technology, there exists a need for more convenient and reliable instrumentation for the detection of unintentional or casual pollutant materials in the areas where people live and work. Two important and typical classes of atmospheric pollutants falling within these comtaminated environment categories are the organophosphorus compounds used for pest control, chemical warfare, and certain industrial purposes, and the oxides of nitrogen, particularly nitrogen dioxide (NO.sub.2), that are unintentionally admitted into the atmosphere from automotive exhaust, combustion stacks, high temperature combustion events and other sources. Nitrogen dioxide is, for example, a known constituent of some ingredient decomposition reactions and, in fact, can be used as a characteristic indicator associated with the progression of these decomposition reactions and ingredient status.
A significant portion of organophosphorus contaminants are found to contain either the phosphoryl or thiophosphoryl group of atoms. Since diisopropyl methylphosphonate (DIMP) is a phosphoryl containing compound having low-toxicity, and significantly documented properties, it is convenient for use as a model organophosphorus gaseous compound in the description of the present invention.
The coated bulk-wave piezoelectric quartz crystal microbalances and surface acoustic wave transducers have recently been considered candidate technologies for detecting and measuring such pollutants as nitrogen dioxide, NO.sub.2, and the organophosphorus compounds. These devices have been reported in various publications including the reports of E. P. Scheide and G. G. Guilbault in "Analytical Chemistry", volume 44, pages 1764-1768, 1972; W. M. Shackelford and G. G. Guilbault, in "Analytica Chimica Acta", volume 73, pages 383-389, 1974; Y. Tomita and G. G. Guilbault, in "Analytical Chemistry", volume 52, pages 1484-1489, 1980; G. G. Guilbault, Y. Tomita, and E. S. Kolesar, Jr. in "Sensors and Actuators", volume 2, pages 43-57, 1981; G. G. Guilbault, J. Affolter, and E. S. Kolesar, Jr. in "Analytical Chemistry", volume 53, pages 2057-2060, 1981; K. H. Karmarker and G. G. Guilbault in "Analytica Chimica Acta", volume 75, pages 111-117, 1975; and L. M. Webber, J. Hlavay, and G. G. Guilbault in "Mikrochimica Acta," volume 1, pages 351-358, 1978.
Surface acoustic wave detection devices have similarly been reported by A. W. Barendsz, J. C. Vis, M. S. Nieuwenhuizen, E. Nieuwkoop, M. J. Vellekoop, W. J. Ghijsen, and A. Venema in "Proceedings of the IEEE Ultrasonics Symposium", page 586, 1985; M. S. Nieuwenhuizen, A. W. Barendsz, "Electronic Letters", volume 22, pages 184-185, 1986; A. Venema, E. Nieuwkoop, W. J. Ghijsen, A. W. Barendsz, and M. S. Nieuwenhuizen in "IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control", volume UFFC-34, pages 148-154, 1987; M. S. Nieuwenhuizen and A. W. Barendsz in "Sensors and Actuators", volume 11, pages 45-62, 1987; M. S. Nieuwenhuizen, A. Nederlof, A. W. Barendsz in "Analytical Chemistry", volume 60, pages 230-235, 1988. Dectectors based on coated bulk-wave piezoelectric crystal microbalances and surface acoustic wave devices are, however, found to have significant limitations including a notable lack of sensitivity and undesirable response to moisture along with difficulty in reproducing identical measurement results.
The concept of utilizing a chemiresistor or interdigitated electrode electrical resistance structure for monitoring impedance changes caused by a chemical reaction have also been reported in the technical literature by F. W. Kutzler, W. Barger, A. Snow, and H. Woltjen in "Thin Solid Films", volume 155, page 155, 1987. Similarly, epoxy cure monitoring and the interaction of NO.sub.2 and organophosphorus compounds with phthalocyanine films have also been considered as is evidenced by the work of S. Baker, G. G. Roberts, and M. C. Petty in "IEE Proceedings", Part 1, volume 130, pages 260-263, 1983; and by H. Wohltjen, W. Barger, and A. Snow, in "Proceedings of the IEEE International Conference on Solid State Sensors and Actuators", pages 410-413, 1985; by H. Wohltjen, W. Barger, A. Snow, and N. L. Jarvis in "IEEE Transactions on Electron Devices", volume ED-32, pages 1170-1174, 1985; by R. H. Tregold, M. C. J. Young, B. Hodge, and A. Hoorfar in "IEE Proceedings", Part 1, volume 132, pages 151-156, 1985; T. Jones and B. Bott in "Sensors and Actuators", volume 9, pages 27-37, 1986; and P. M. Burr, P. D. Jeffery, J. D. Benjamin, and M. J. Uren in "Thin Solid Films", volume 151, pages L111-L113, 1987.
It is important to note, however, that most of these previous investigations of the chemiresistor and phthalocyanine embodiments, thereof, have focused on the direct current electrical conductivity changes in the chemically active film while only a few investigators have considered the alternating current behavior of these films and, notably, have considered this alternating current behavior only at specific and single alternating current frequencies.
Additional evidence of previous gaseous component identification investigations is also to be found in the patent art including the patent of W. R. Barger et al, U.S. Pat. No. 4,636,767, wherein an interdigitated finger structure and a phthalocyanine film are used in combination with direct current excitation of the detecting cell for the monitoring of gaseous components. Also included in this art is the patent of J. R. Stetter, U.S. Pat. No. 4,670,405, which considers the time response of a plurality of sensor elements taken over time periods measured in tens of seconds and processed in a microprocessor computer.
Also included in the patent art is the detection and measuring system of P. K. Clifford described in U.S. Pat. No. 4,542,640 which includes a plurality of differing semiconductor gas sensor cells and apparatus for measuring their electrical resistance and for processing the resulting signals. In addition, the patent of R. H. Duhlgren et al, U.S. Pat. No. 4,725,733, discloses an optically based detection arrangement for chemical warfare nerve agents, and the patent of E. J. Poziomek et al, U.S. Pat. No. 3,910,763, discloses a chemical reaction based detection arrangement for organophosphorus compounds.